Light waves may be made to carry information by modulating a light source, often a laser source, to change various properties of the light, such as its amplitude, phase, frequency, wavelength, etc. The light waves may be in the visible spectral band, the infrared spectral band, or another region of the electromagnetic spectrum. In some cases, an underlying signal, such as a radio frequency signal, may be modulated via amplitude, phase, or frequency modulation, or any combination thereof, and the light source may be modulated by the underlying signal. Optical receivers receive the light waves and measure properties or variations of the light wave, such as the amplitude, phase transitions, and the like, from which the underlying signal and the information may be recovered.
Phase modulation of light signals may convey useful information. Information encoded in phase modulation may include transmitted communication data, or may include other information such as information about the source of the optical signal, interaction of the optical signal with an object, the optical channel through which the optical signal traveled, and/or objects with which it interacted. Compared to typical amplitude modulation receivers, phase modulation receivers can be significantly more complex, requiring precision optics, local oscillators, Fiber Bragg Gratings (FBG), and/or delay line interferometers (DLI), etc.
A receiver for modulated light waves should collect signal from a large enough area that the acquired signal power is high enough for accurate detection. Conventionally, a telescope may be aimed at the light source and the cross sectional area of the telescope, or aperture, may determine how much signal power is collected and concentrated (e.g., focused) at a receiver. When such light is phase modulated, optimal reception occurs if all the light rays (across the cross-section of the telescope) arrive at the detector in unison as a single wavefront, maintaining alignment of the original phase relationships of the light rays. Wavefront correction may be required in conventional light-focusing systems, e.g., if the light rays have propagated through varying media along the way, or were skewed, delayed, aberrated, or the like, as is typical for light waves traveling some distance through the atmosphere. Such systems may use adaptive optics to attempt to correct the light rays to their original phase relationships, but such are complex, fragile, and costly.